Photoresponsive magnetization reversal in green fluorescent protein chromophore based diradicals

We present density functional theory based calculations on six diradicals in which imino nitroxide (IN) and IN or oxo- or phospha-verdazyl radical centres are linked by fragments of structures called cyan, blue and green fluorescent protein, respectively. In the latter two variants the indolyl fragment of cyan fluorescent protein (CFP) is replaced by phenyl and phenol moiety to obtain blue fluorescent protein Y66F GFP (BFPF) and green fluorescent protein (GFP). The photoinduced cis–trans isomerization of these diradical substituted chromophores is accompanied by changes from antiferro- to ferro-magnetic spin coupling on the radical centres. The calculations in this work provide predictions of the change of magnetic response to photo excitation. The estimated magnetic exchange coupling constants associated with cis–trans isomerization of the chromophores can be useful parameters in designing radical substituted isomeric fluorescent chromophores. Upon irradiation with light of appropriate wavelength, the dark trans diradicals turn into their fluorescent cis isomers. Therefore, photoinduced magnetic crossover from antiferromagnetic to ferromagnetic regime associated with the change in color would be noticed in all three cases. This is a novel observation in case of the systems with GFP chromophore and its variants. These diradicals are potentially very useful in different applications and biocompatibility of such systems makes them prospective in different in vivo treatments. Moreover, change in color associated with magnetic crossover for these diradicals increases their suitability as biological taggers.